In the case of a separator film, at first the film is manufactured by an extruder and then wound by a winder. An oscillation winding method is often used when winding. In the oscillation winding method, the separator film is traversed in the wide direction thereof while winding, as described in JP 10-212059 A (Patent document 1). Therefore, if the separator film has an uneven thickness in the wide direction thereof, the thick portions and the thin portions thereof are superposed alternately so that the film can be wound under even pressure and the winding roll can become a good shaped. Then a supply roll is put to a slitter, the separator film is supplied from the supply roll and slit into films having predetermined widths.
However, if the separator film is wound by the oscillation winding method just after being manufactured, the separator film includes widthwise opposite end portions which shrink significantly as time passes. It is believed that the reason relates to the material and winding conditions of the separator film. The separator film is made from synthetic resin such as porous polyolefin. If the separator film is wound by the oscillation winding method just after being manufactured, the widthwise center portion thereof is wound tightly, but the widthwise opposite end portions thereof are not wound tightly. Then the separator film is cooled with the state of the winding roll. Therefore, the widthwise opposite end portions shrink significantly.
Therefore, when the supply roll is put to a slitter and the separator film is supplied from the supply roll, the widthwise opposite end portions decrease in length by the shrinkage phenomenon to result in the difference in length between the widthwise opposite end portions and the widthwise center portion. Accordingly, when the separator film is engaged with a guide roller, only the widthwise opposite end portions thereof tense by the tension but the widthwise center portion thereof does not tense. In this connection, the separator film must be loosened. As a result, the device has a problem that the separator film gets wrinkles.
If not only the separator film but also another web is wound by the oscillation winding method, the widthwise opposite end portions thereof must shrink partly. Therefore, when the web is supplied from the supply roll and engaged with the guide roller, the device has similarly a problem that the web gets wrinkles.
It is, therefore, an object of the present invention to provide a slitter in which the web cannot get wrinkles when the web wound by the oscillation winding method is supplied from the supply roll and engaged with the guide roller.    Patent document 1: JP 10-212059 A